# Peptide Reconstitution Calculator

Enter the amount in the vial and how much bacteriostatic water you added. The calculator gives the concentration, then the volume in mL and U-100 units that holds the amount you enter, rounded to a marking you can actually read.

Canonical: https://bestpeptidecalculator.app/reconstitution-calculator
Last reviewed: 2026-11-08

Interactive calculator on the page: reconstitution. It runs in the browser and never suggests an amount.

## What reconstitution means in numbers

Reconstituting a vial means dissolving its powder in a known volume of liquid. Adding liquid never changes how much is in the vial. It only spreads that amount through a volume, and the concentration is the result.

`concentration = amount in the vial ÷ liquid added`

More liquid gives a lower concentration, so each amount takes up more volume. That is easier to measure on a syringe, at the cost of a larger volume.

## Step by step

1. Read the total amount on the vial label (for example 10 mg).
2. Note the exact volume of liquid you added (for example 3 mL).
3. Concentration: 10 mg ÷ 3 mL = 3.333 mg/mL (3,333 mcg/mL).
4. Pick an amount and convert it to the same unit (for example 500 mcg).
5. Volume: 500 mcg × 3 mL ÷ 10,000 mcg = 0.15 mL, which is 15 units on a U-100 syringe.

The calculator above does exactly these steps and shows them under “How this was calculated”.

## How the liquid volume changes the numbers

| Liquid added | Concentration | Volume for 250 mcg | U-100 units |
| --- | --- | --- | --- |
| 1 mL | 5 mg/mL | 0.05 mL | 5 units |
| 2 mL | 2.5 mg/mL | 0.10 mL | 10 units |
| 2.5 mL | 2 mg/mL | 0.125 mL | 12.5 units |
| 3 mL | 1.667 mg/mL | 0.15 mL | 15 units |

A volume that falls between markings, like 12.5 units on a syringe marked every 2 units, can’t be read exactly. The calculator shows the nearest marking and what it actually holds. There is more on choosing a liquid volume in [how much bacteriostatic water to add](https://bestpeptidecalculator.app/guides/how-much-bac-water).

## Common mistakes

| Mistake | What happens | Fix |
| --- | --- | --- |
| Mixing mg and mcg | Results off by 1,000× | Pick the unit next to each number; the calculator converts exactly |
| Treating syringe units as IU | Wrong amount for IU products | Units are a volume marking (100 per mL); IU needs a factor |
| Using the planned liquid instead of the actual | Concentration is wrong for the whole vial | Enter the volume you actually added |
| Reading between markings | Small amounts drift by 5% or more | Use a finer syringe or more liquid; the calculator warns you |

## Working backwards from a volume

Switch to “Amount in a volume” to see what a measured volume holds: 20 units of a 2.5 mg/mL solution is 0.2 mL, which holds 0.5 mg (500 mcg).

`amount = volume (mL) × amount in the vial ÷ liquid added`

## Frequently asked questions

### How much bacteriostatic water should I add?

There is no single right volume: the amount in the vial stays the same whatever you add. More liquid makes each amount a larger, easier-to-measure volume. The calculator shows how any choice changes the volume and units. See [how much bacteriostatic water to add](https://bestpeptidecalculator.app/guides/how-much-bac-water).

### How do you calculate peptide reconstitution?

Divide the amount in the vial by the liquid added to get the concentration. Then divide the amount you choose by the concentration to get the volume in mL. Multiply mL by 100 for U-100 syringe units.

### Does adding more water change how much is in the vial?

No. The total amount is fixed. Liquid only changes the concentration, so the same amount takes up a different volume.

### What is the difference between bacteriostatic water and sterile water?

Bacteriostatic water is sterile water with a preservative (usually 0.9% benzyl alcohol) that slows bacterial growth, so a vial can be entered more than once. Sterile water has no preservative. The math is the same for both. See [bacteriostatic water explained](https://bestpeptidecalculator.app/guides/bacteriostatic-water-storage).

### How long does a reconstituted peptide keep?

It depends on the compound and its supplier’s data. This calculator doesn’t estimate it: check the product’s documentation.

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Research and educational use only. Not medical advice. Best Peptide Calculator is powered by Peptide Library (https://peptidelibrary.app).